1. Field of the Invention
This invention relates to a green light-emitting phosphor for a projection cathode ray tube, more particularly to a mixed green light-emitting phosphor prepared by mixing a plurality of green light-emitting phosphors with a predetermined weight ratio, to improve the brightness and the color tone of the fluorescent screen of the projection cathode ray tube.
Description of the Prior Arts
Conventionally, typical green light-emitting phosphors for projection cathode ray tubes are Y.sub.3 Al.sub.5 O.sub.12 :Tb and Y.sub.3 (Al,Ga).sub.5 O.sub.12 :Tb, which have excellent characteristics in persistence and brightness saturation but have inferior characteristics in brightness and chromaticity. Another prior green light-emitting phosphor used for projection cathode ray tubes is LaOCl:Tb, which has not good saturation characteristics of temperature and chromaticity in the range of high current.
It is desirable that a green light-emitting phosphor for projection cathode ray tubes must have a feature of high brightness from the range of low current to high current, excellent saturation characteristics of brightness in the high current range, and a longevity of afterglow characteristics. To this end, recently a mixture of phosphors having unique characteristics, with a predetermined weight ratio has been used to compensate for a respective disadvantages therebetween, so as to improve various characteristics of the phosphors applicable to the fluorescent screen.
For instance, Japanese Patent Laid-Open, SHOWA 63-381 discloses a mixture of green light-emitting phosphors consisting of a red phosphor P.sub.27, Zn.sub.3 (PO.sub.4).sub.2 :Mn and a green phosphor P.sub.1, Zn.sub.2 SiO.sub.4 :Mn to make an orange color fluorescent screen for cathode ray tube, which can enhance the characteristics of long afterglow.
Another Japanese Patent Laid-Open, HEISEI 1-215884 discloses a mixture of green light-emitting phosphors consisting of a LaOCl:Tb phosphor and a Y.sub.2 SiO.sub.2 :Tb, phosphor having a predetermined mean particle diameter which phosphors are mixed with a selective weight ratio. This Japanese patent document sets forth a green phosphor which can improve brightness of an image projected on the screen under excitation by an electron beam from low current to high current range resulting from the compensation of respective characteristics of each phosphor.
Japanese Patent Laid-Open HEISEI 1-284584 discloses a luminescent element for a color display CRT which is a light blue or a white phosphor created by mixing a ZnS phosphor activated with Cu, Au, or Al and a Y.sub.2 O.sub.2 S phosphor activated with Eu by adding a ZnS phosphor activated with Ag, a Y.sub.2 O.sub.2 S phosphor activated with Tb, or a Gd.sub.2 O.sub.2 S phosphor activated with Tb in a predetermined weight ratio. Such mixture of phosphors provides the same color tone at a low current and at a high current and improves the brightness at a high current density.
However, above Japanese Patent Laid-Open SHOWA 63-381 and HEISEI 1-284584 relate to a mixture of phosphors for fluorescent screen having an orange color, a light blue color, or a white color. Japanese Patent Laid-Open HEISEI 1-215884 concerns a mixture of phosphors for fluorescent screen consisting of LaOCl activated with Tb and Y.sub.2 SiO.sub.5 activated with Tb, which only discloses an improvement of the brightness over all of the range of current without providing various luminescent characteristics of the mixture of each phosphor.